Tens of thousands of Iraqi volunteers and hardened Shia militia are moving north out of Baghdad as the government seeks to wrestle back control of towns over-run by Sunni insurgents.
Even as the militants' advance slowed, the capital has been placed on a war footing, with a heightened security presence. Leading Shia politicians are imploring their constituents to take up arms and fight the terrorist threat.
The militants, joined by loyalists of ousted president Saddam Hussein, have met little resistance in Sunni areas, where there is resentment against the Shia-led government in Baghdad, raising concerns about the fragmentation of Iraq along sectarian and ethnic lines.
The Islamic State of Iraq and the Levant (known as Isis) posted images purporting to show the execution of hundreds of Iraqi army soldiers, inflaming passions among the Shia community. The pictures could not be verified. Amid heightening sectarian tensions in the capital, there are many as yet unfounded rumours of reprisals against Sunni Muslims.
Young men have been flooding into recruitment centres in the capital and other Shia-majority cities, such as Karbala, as they heed a call from Grand Ayatollah Ali al-Sistani, the country's most senior cleric, to enlist in the state's security forces but to refrain from inflaming sectarian divisions.
Ammar al-Hakim, the leader of the Shia Islamic Supreme Council of Iraq, was seen in military fatigues rather than his usual clerical clothes.
"We are ready to die," said Ali, a young recruit brandishing a green Shia banner. "We don't want Isis in Iraq - no mercy for terrorism."
Shia militia groups are also moving north to bolster the army effort as politicians fear that US military support may not arrive.
Ahmed al-Kinani, a spokesman for Asa'ib Ahl al-Haq, said the hardline Shia group has deployed "thousands of fighters" to Samarra in response to the grand ayatollah's call to help defend religious sites from the advancing insurgent forces.
The Asa'ib forces, who gained military experience fighting US forces during the occupation, are set to move forward to join the battle against Isis fighters if ordered, he added.
An army captain in Baghdad said volunteer troops would receive training at Camp Taji, a large military base 30km north of the capital, before heading out to join the army or federal police.
Raghad Hussein, Saddam's eldest daughter, has expressed support for the uprising against the Maliki government. Speaking to Al-Quds al-Arabi newspaper, she lauded Izzat Ibrahim al-Douri for leading former Ba'ath officers in bolstering the Isis forces' advance.
Meanwhile, reports emerged that Mr al-Douri's son was killed in an air strike on a mosque in the town of Fallujah, one of Isis's main gains over the past week.
Since the US occupation of Iraq in 2003 which toppled Saddam and gutted his ruling Ba'ath party, the country has been mired in a sectarian power struggle between disempowered Sunnis and the Shia majority.
Isis stunned the world by taking over Mosul, Iraq's second-largest city, on Monday. In the past five days it seized about 10 per cent of Iraqi territory, including much of oil-rich Nineveh province as well as Tikrit, Saddam's home town, and the important oil refining town of Baiji.
Government officials have said the refinery remains under state control.
Iranian president Hassan Rouhani said his country would be willing to work with the US, its long-time rival for influence in Iraq and the Middle East, but said it was unlikely it would deploy forces to the country.
Washington has also stalled on offering military action without a "political plan" that showed Mr Maliki's government would work to ease sectarian tensions.
However, the US on Saturday moved an aircraft carrier into the Persian Gulf, giving it much greater capacity to conduct air strikes in northern Iraq against Islamist insurgents.
The Pentagon said the USS George HW Bush was moving from the Arabian Sea into the Gulf and was being accompanied by a guided-missile cruiser and a guided-missile destroyer. The deployments would provide "additional flexibility should military options be required", said Admiral John Kirby.
But with Kurdish peshmerga forces now reinforcing the borders of their semi-autonomous region, Sunni regions in militant hands and Shia volunteers rushing to defend Baghdad, the country appears to be collapsing into sectarian cantons.
In Isis-captured areas, many Ba'athist groups are believed to have supported the fight, and there are signs that Sunnis are beginning to see the offensive as a Sunni revolt against Mr Maliki's government.
The Muslim Scholars Association of Iraq, a Sunni group, called on the militants, which it described as "revolutionaries" to halt their advance in Baghdad to avoid stirring worse sectarian strife.
Earlier this week, Isis said its fighters would take Baghdad and then head to the mostly Shia south to attack Najaf and Karbala, home to some of the most important Shia shrines.
"It is their right to take Baghdad because the ruling government there is a source of oppression and criminality," the group said. "But we warn the revolutionaries against going to Najaf or Karbala . . . as it would bring failure and change the goal from supporting the oppressed to sowing discord."
The peshmerga on Saturday sent truckloads of fighters and tanks to the disputed Diyala province, a mix of Sunnis, Shia and Kurds.
"Isis cannot go any further towards Baghdad so now it is trying to wage war on Kurds," said Issa Berkati, a captain escorting the reinforcements. "We will die before we let them take part of Kurdistan."
Additional reporting by Geoff Dyer in Washington
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